Halftoning is a process by which continuous tone images are approximated by a pattern of pixels that can achieve only a limited number of discrete intensities. An example of this is the rendering of gray tones with black and white pixels, such as in a newspaper photograph. A halftone pattern is made up of a region of pixels referred to as a halftone cell. In conventional digital halftoning (e.g., halftoning that uses rational tangent angles), a halftone cell includes a specific, repeatable pattern. The discrete number of tonal levels of a halftone pattern depends upon the number of pixels in the halftone cell and the number of exposure levels or dot sizes for each pixel.
Imaging systems often require some type of calibration to achieve a desired target response. Halftone calibrations typically involve converting the halftone description from a threshold basis that is most commonly used to a system that includes the halftone patterns for all gray levels. This can be visualized as a three-dimensional (3-D) lookup table (LUT) in which a calibrated version of the 3-D LUT can be created by rearranging the halftone patterns based on a transfer function LUT. This reorders the patterns of the halftone such that some patterns are replicated while others are deleted. The final calibrated 3-D LUT is then converted back into a threshold representation that is commonly used. This is a very time consuming process, especially if the array is very large (e.g., as is the case of stochastic halftones with full page width threshold arrays).
Accordingly, an improved halftone calibration mechanism is desired.